In general, a torque transmission mechanism having a torque limiting function is known. Such a torque transmission mechanism is disclosed in U.S. Pat. No. 6,231,012, for example.
A torque transmission mechanism disclosed in the aforementioned U.S. Pat. No. 6,231,012 includes an input shaft, an output shaft, and an intermediate gear portion that connects the input shaft to the output shaft. The intermediate gear portion is provided with an input-side gear and an output shaft gear on both ends of a shaft member provided with a notch in its outer periphery, respectively. When the input torque of at least the predetermined value is applied to the shaft member of the intermediate gear portion, a notched portion is broken (fractured). Thus, in the intermediate gear portion, rotation of the input-side gear is no longer transmitted to the output-side gear. Consequently, transmission of the input torque of at least the predetermined value to an output shaft side is prevented.